Help needed

Hi guys
I wish to install GT 12AX7-M (Mullard) tubes inside my SVT3 Pro ... which would be the best position ? I was thinking 2° and 3° position and eventually one in the driver tube section .. Could be this a good idea ? 1° position I'm stuck with a 12AX7-C
Anyone know 12AX7-U serbian made ?
Any advice is well accepted ...